1. Field of the Invention
The invention pertains to the field of variable cam timing systems. More particularly, the invention pertains to variable cam timing systems of the kind having phasers for varying the radial disposition of a camshaft relative to its drive means (sprocket or drive gear).
2. Description of Related Art
Traditionally, the camshaft (or, in a multiple camshaft engine, camshafts) of an internal combustion engine, which actuates the intake and/or exhaust valves, is connected to the crankshaft, which receives the force from the pistons, by a timing chain, belt or gear arrangement driving sprockets, pulleys or gears, respectively, on the ends of the shafts. The relative timing of the camshaft(s) and crankshaft in such a system is fixed, and must be chosen to be tailored to power or economy at a given engine speed or load condition. This is inherently a compromise, as an automobile engine does not, obviously, always run at the same speed or load, and a given car owner might desire either power or economy at different times. The demands of emissions control complicate matters further.
This has given rise to Variable Cam Timing (VCT) systems, where the timing of the valves relative to the crankshaft can be changed by altering the relative rotational positions of the camshaft(s) and crankshaft. One of the more successful systems for VCT involves using a device called a xe2x80x9cphaserxe2x80x9d to allow the camshaft sprocket, which is linked to the crankshaft by the timing chain, to shift angular position relative to the end of the camshaft. Typically, the phaser is a coaxial arrangement of an outer housing which forms the sprocket (or pulley or gear) and an inner rotor fixed to the camshaft. The angular position of the rotor and housing can be shifted by fluid pressure acting on pistons or vanes on the rotor inside cylinders or chambers formed in the housing.
The xe2x80x9cvane phaserxe2x80x9d setup is commonly used in VCT systems, and will be used in the examples in this disclosure, although it will be understood that the method of the invention will work with other forms of phasers known to the art. Butterfield and Smith, U.S. Pat. No. 5,172,659, xe2x80x9cDifferential Pressure Control System for Variable Camshaft Timing Systemxe2x80x9d, assigned to BorgWarner Inc., shows a vane phaser system which uses the inherent torque reversals in the camshaft caused by the actuation of the valves to move the vane from one position to another. Fluid is led from one side of each vane to the opposing side through a valve. When the valve is open, the rotor is free to oscillate, the fluid passing freely from one side of the vane to the other. When the valve is closed, the fluid cannot flow, and the vane is held in position. By opening the valve while the torque reversal is acting to move the camshaft in the desired direction, then closing the valve, the camshaft is allowed to move, then held in place by the fluid on each side of the vane.
A number of U.S. patents show phasers which have mechanical locking mechanisms. The locking of the phaser is most often provided to prevent unwanted phase shifts during periods of high torque reversals, when the actuating force of the phaser is not sufficient to hold the selected timing, as during engine start-up, when engine oil pressure is low, reducing the available pressure to activate the phaser, the oil in the phaser may have leaked away, and the erratic engine operation can lead to dramatic forces on the cam. The following patents show different means of locking a phaser in place.
Simpson, U.S. Pat. No. 6,250,265 xe2x80x9cVariable Valve Timing With Actuator Locking for Internal Combustion Enginexe2x80x9d, assigned to BorgWarner Inc, shows a vane-type phaser with a locking mechanism which is released by engine oil pressure, so as to lock the phaser when engine oil pressure is low.
Trzmiel, et. al, U.S. Pat. No. 6,053,138, xe2x80x9cDevice for Hydraulic Rotational Angle Adjustment of a Shaft Relative to a Drive Wheelxe2x80x9d, assigned to Porsche AG and Hydraulik Ring GmbH, also uses a hydraulic brake arrangement.
Muir et. al, U.S. Pat. No. 5,031,585, xe2x80x9cElectromagnetic Brake for a Camshaft Phase Change Devicexe2x80x9d, assigned to Eaton Corporation, uses an electromagnetic clutch to lock the phaser.
Suga, et. al, U.S. Pat. No. 5,117,785, xe2x80x9cValve Timing Control Device for Internal Combustion Enginexe2x80x9d, assigned to Atsugi Unisia Corporation, uses a cam or wedge locking system.
All mechanical systems have one or more resonant frequencies, where the characteristics of the system change, sometimes abruptly, with the frequency of actuation. In the case of a valve timing system for an internal combustion engine, the resonant frequencies of the camshaft, crankshaft and timing chain/belt/gears will all combine into a complex set of reactions which can lead to excessive noise or vibration at specific engine RPM.
If an engine is fitted with a VCT phaser, the resonant characteristics of the timing system will change, depending on whether the phaser is locked (i.e. the rotor and housing are acting as a unit) or unlocked (the rotor and housing can rotate independently to some extent). The method of the invention uses this alteration in characteristics to minimize the effects of resonance, by locking or unlocking the phaser as a resonant point in the engine RPM is approached.